1. The Field of the Invention
The present invention relates to improvements in power factor control, and to systems embodying said improved power factor control; in particular, an LED lamp.
2. The Relevant Technology
Power factor control is considered desirable to mitigate the effects of high harmonic currents drawn from the mains supply line by non-linear loads such as rectifier and smoothing capacitor input stages.
One method of achieving this is to employ a boost converter charging a storage capacitor to a voltage higher than the peak voltage of the incoming supply, and to shape the average current waveform to match the incoming supply voltage waveform so that the power factor drawn is close to unity. However (see FIG. 1) such prior art designs presently use a hard switched transistor device T1 to connect the boost inductor L1 periodically to the negative end of the input full wave rectifier bridge and build up a current in this inductor then switch the transistor T1 off so that the current flowing in the inductor then charges the storage capacitor C2 to a higher voltage. The rectified average current drawn from the line is measured in a small resistance R3 and compared against a proportion of the rectified input supply voltage at the junction of R1 and R2.
As switching is done at a frequency substantially higher than the incoming supply frequency the current waveform can be made to average out to match the supply waveform. Many integrated circuit control schemes exist to provide these functions. There are several modes of operation, where the current may be continuous through the inductor, or discontinuous, or critical where the switching on of T1 is done at the instant that the current through the inductance reaches zero. However all modes have an undesirable effect in that the switching of the transistor should be fast to minimise switching loss (where the device momentarily supports both voltage across it and current through it) and this results in Electromagnetic Interference (EMI) occurring which must be suppressed and filtered so that it cannot conduct onto the supply lines or radiate out into the surroundings.
In addition to dissipative snubber circuits (transient voltage suppressers) across the switching device and/or boost diode, this usually requires a relatively complex and expensive EMI filter to be interposed between the AC supply and the device, and often the provision of metallic screening around it. The output of such a boost converter is high voltage DC which can then be used in a load resistance as shown or be converted to low voltage high current output as desired using any one of several well-known DC-DC converter circuits, and which can add their own EMI component.
A method of zero voltage switching has been proposed (U.S. Pat. No. 5,180,964) for a DC-DC boost converter (FIG. 2) which provides this zero voltage switching function, however this has been employed only for a simple DC-DC boost converter and not in power factor control of an AC supply, and has some disadvantages. In FIG. 2 the addition of current sensing means to detect the zero current in the inductor L1 by a secondary winding L2 on L1 requires an isolated, but well coupled winding which adds to cost and winding difficulty.